1. Field of the Invention
The present invention relates to a method for fabricating deposited organic films that are two-dimensionally and periodically arranged on a III-V group compound semiconductor substrate.
2. Description of the Prior Art
Conventional monomolecular or deposited organic films include Langmuir Blodgett films (hereafter referred to as "LB films") and self-assembled monolayers (Abraham Ulman: An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly, Academic Press 1991).
An LB film is formed by developing on a water surface, as a monomolecular film, amphipathic molecules including hydrophilic functional groups and hydrophobic atomic groups (an L film). The film is transferred onto a solid substrate, and several such films are deposited thereon, according to the process named after Langmuir and Blodgett. A self-assembled monolayer is obtained by allowing the functional groups at the terminals of molecules to be chemically adsorbed by the atoms constituting the substrate. This film is called the "self-assembled monolayer" because, due to the relevant adsorption mechanism, only monomolecular films are self-organized and formed on the substrate.
Films can also be accumulated by selecting the type of the terminal group away from a formed self-assembled monolayer. These monomolecular films form a two-dimensional monomolecular aggregate due to the Van der Waals between the molecules, and these methods can be used to manufacture a periodic array of molecular packing, that is, two-dimensional crystals. This feature can be used to construct electronic and optical devices.
Since an LB film is formed by transferring a film developed on the water surface, onto the substrate, using the difference between the hydrophobic and hydrophilic properties of the film and the substrate, the crystallinity of the film is primely determined when the film is expanded and compressed. Thus, the crystallinity of the film does not depend on the substrate materials, and the film can be formed on any substrate. The interaction between the substrate and the monomolecular film, however, is very weak due to the nature of the LB film, thus the film lacks the acid- and alkali-resistance and durability required to construct complicated devices.
On the other hand, the self-assembled monolayer does not have the above disadvantages, but due to the use of the chemical adsorption between the functional groups of molecules and the substrate, their range of combinations is limited. Monomolecular films have thus been implemented on substrates of silicon oxide, aluminum oxide, silver oxide, mica, gold, copper, or GaAs. For GaAs substrates, only self-assembled monolayers are obtained by treating the substrate with hydrochloric acid solution so as to provide arsen terminated surface, then coating it with a molten liquid of organic molecules including SE groups in a nitrogen atmosphere, and holding it at about 100 C for 5 hours.
Since, however, the self-assembled monolayer uses an As-terminated surface on GaAs, so the surface is amorphous and the two-dimensional crystallinity of a self assembled monolayer obtained has been very low, that is, the quality of this film has been lower than that of the LB film. The film quality directly compared to quantum efficiency that provides a functional characteristics of that monomolecular film, and thus contributes to producing substantial adverse effects in fabricating devices. Furthermore, despite the large number of advantages of the self-assembled monolayer compared to LB film, there have been no examples where a multilayer of self-assembled monolayers has been arranged on a III-V group compound semiconductor substrate; instead, self-assembled monolayers have been used only on silicon oxide film or a gold substrate. A technique that allows the formation of multilayers while preserving the numerous advantages of the self-assembled monolayer is very important because it enables the multilayers to include characteristics that cannot be implemented by more complicated monomolecular films and also enhance functional performance.
Since the self-assembled multilayers heretofore obtained have been formed on an amorphous silicon oxide film despite the use of monocrystal silicon for the substrate, such multilayers may not have an ordered structure along their lateral orientation or may have two-dimensional domain structures with a large number of pin holes.
To solve this problem, attempts have been made to provide a substrate the surface of which is flat on an atomic level and the atoms of which are periodically disposed (in a .sqroot. 3.times.23 structure), with the multilayer then formed on this substrate. In these attempts, a single crystal or gold or an epitaxial thin film substrate of gold formed on a mica cleavage plane has been employed, using the EB deposition method. In these cases, however, since the surface of the substrate has a reconstructed (i.e., .sqroot. 3.times.23) structure with a high atom packing density instead of the bulk structure, as well as a long-period, so-called "Herringbone" structure, excess gold atoms may be protruded and thus depressions may be formed on the substrate surface when a self-assembled mono- and multilayer are fomred thereon. Therefore the film itself has domain structures of only several 10 nm in size. Consequently, these organic films have degraded insulating and I-V characteristics. To improve the performance of organic films obtained along these lines, it is necessary to producean organic superlattice multilayer that is flat and dense onan atomic level and which has a two-dimensional order without pin holes. There is demand both for such an organic film and for a method to fabricate it.